A semiconductor device is manufactured by repetitively performing a process for transferring a pattern formed on a photo mask onto a wafer through a lithography process and an etching process. In a semiconductor manufacturing device, since a yield of a semiconductor device is influenced by a failure in a manufacturing process such as an etching process and generation of alien substances, it is important to inspect/measure a pattern on wafer during a manufacturing process to detect an occurrence of an abnormality or a failure as early as possible. Therefore, in a current semiconductor device manufacturing line, a technique for inspecting/measuring a state of a pattern formed on a wafer during a manufacturing process plays an important role. Conventionally, an inspection/measurement technique is mostly based on an optical microscope, but recently an inspection/measurement apparatus based on an electronic microscope is widely being spread to cope with miniaturization of a semiconductor device and sophistication of a manufacturing process. Particularly, in managing a dimension of a semiconductor circuit pattern, a length measuring scanning electron microscope (SEM) based on an electronic microscope is currently used as a quality managing means which is indispensable to a manufacturing process. In managing a dimension of a fine pattern, high surface resolution, high measuring accuracy, and high reproducibility are required, and it is also indispensable to suppress damage to a circuit pattern when measured. In order to satisfy such requirements, a primary electron beam is accelerated at high energy and is decelerated, before being incident to a specimen, at a retarding voltage applied to a specimen containing a semiconductor pattern which is a measurement target.
However, if a surface of a semiconductor device containing an insulator is scanned by a primary electron beam, an electrification state of a surface may change depending on a scanning condition. Therefore, the following faults may occur: (1) a detection rate of a secondary signal emitted from a pattern portion fluctuates, and an abnormal contrast occurs in a secondary signal image; and (2) a scanning position of a primary electron changes depending on a change of electrification, and measurement accuracy and reproducibility of a pattern dimension may deteriorate. Therefore, it is important to detect an electrification state of a semiconductor device and to feed it back to a measurement condition before measurement and to maintain an electrification state of a semiconductor device surface during measurement.
Also, in inspecting a semiconductor device, it is highly required to detect an electrical characteristic fault such as conduction and non-conduction which an optical inspection apparatus is difficult to detect, and thus an electron beam inspection apparatus comes into wide use. An electron beam inspection apparatus detects an electrical characteristic fault by charging a circuit pattern formed on a wafer surface and using a contrast actualized by it. It is called a potential contrast technique, and it is a useful means to detect an electrical characteristic fault of a semiconductor device. In order to detect such a fault with higher sensitivity, it is indispensable to appropriately charge a semiconductor device.
As a technique for controlling an electrification state of an inspected/measured specimen at high accuracy, Japanese Patent Laid-open Publication no. 2000-208579 discloses a technique that a desired voltage is applied to an electrode called an electrification control electrode disposed opposite to a specimen, and an electron beam is irradiated to a specimen from a secondary electron source, which is different from an electron source for a primary electron beam, to control electrification potential of a specimen. PCT Publication no. WO2003/7330 discloses a technique that surface potential is measured by using a surface potential meter (SPM), and a preliminary electrification/destaticization condition or an inspection/measurement condition of a semiconductor device surface is optimized based on the result.
A principle of controlling electric potential of a specimen surface using an electrification control electrode will be described with reference to FIG. 4. FIG. 4 is a diagram illustrating a disposition relationship between an inspected specimen and an electrification control electrode when a contact hole having a conduction defect formed therein is used as an inspection specimen. An inspected specimen has a structure in which a SiO2 layer 405 is formed on a Si substrate 404, a contact hole is formed, and metal is embedded inside the hole as shown in a cross-sectional view of a wafer 400 of FIG. 5.
An electron source 10 and an electrification control electrode 407 are disposed above the wafer 400. The electrification control electrode 407 has a hole which a primary electron beam and a secondary charged particle pass through. Various lenses are disposed between the electron source 10 and the electrification control electrode 407 but are not shown in FIG. 4. A reference numeral 17 denotes a reflecting plate 17, and a reference numeral 411 denotes a secondary electron detector. Retarding potential 406 is applied to the wafer 400, and predetermined potential (electrification control electrode potential) 408 based on the wafer 400 is applied to the electrification control electrode 407. The primary electron beam 410 arriving at the wafer interacts with the wafer to generate the secondary charged particle.
In a potential contrast technique, a difference between a normal portion and a defective portion is detected as a contrast difference of a potential contrast image. A contrast difference results from the fact that an electrification potential difference occurs since a normal portion and a defective portion are different in electric resistance, and as a result, there occurs a difference in number of secondary electrons detected. Therefore, in order to detect a fault by a potential contrast technique, there is a need for electrically charging a wafer to make a sufficient electrification potential difference between a normal portion and a defective portion. A wafer surface can be electrically charged to either of (1) a positive voltage contrast (PCV) and (2) a negative voltage contrast (NVC), and a polarity of electrification depends on a structure of a wafer which is an inspection target or an inspection condition. Here, a principle of a wafer negative voltage contrast (NVC) will be described below.
A potential distribution is formed between the electrification control electrode 407 and the wafer 400 by electric potential 408 of the electrification control electrode 407 and electric potential 406 of the wafer 400. A change of a potential distribution along an optical axis of a primary electron beam is indicated by a curve 413 of FIG. 4. As indicated by the curve 413, in the potential distribution, there exists a position where electric potential is minimum (position where electric potential becomes negative maximum), and a potential difference 412 between electric potential at the position (hereinafter, minimum potential) and wafer surface potential functions as a potential barrier of a secondary signal emitted from a wafer surface.
In the secondary signal 409 emitted from the wafer 400 by irradiation of the primary electron beam, an element that kinetic energy is higher than the potential barrier 412 goes over the barrier and is detected by the detector 411. Meanwhile, an element of the secondary signal that kinetic energy is lower than the potential barrier 412 returns to the wafer surface 414 and electrically charges the wafer to a negative. In order to electrically charge the wafer to a positive, a voltage applied to the electrification control electrode 407 is appropriately adjusted so that the number of secondary electrons emitted from the wafer can be greater than the number of electrons contained in the primary electron beam which arrives at a specimen. As a result, the wafer surface is electrically charged to a positive.
U.S. Pat. No. 6,586,736 B1 discloses an invention which applies an electrification control electrode described above. According to an invention disclosed in U.S. Pat. No. 6,586,736 B1, if an incident angle of a primary electron beam to a specimen is deflected (strays) from an electron beam optical axis, secondary charged particles which return to a specimen surface are increased, so that it is difficult to control a potential distribution of a specimen surface. In order to resolve the problem that an incident angle of a primary electron beam is deflected, U.S. Pat. No. 6,586,736 B1 employs a three-electrode structure as an electrification control electrode, sets an electrode (i.e., lowest electrode) proximal to a specimen to the same voltage as a retarding voltage, divides an intermediate electrode into left and right centering on an optical axis, and changes voltages applied to the divided electrodes left and right.